1. Technical Field
The disclosure relates to viscous or shear fluid torque coupling devices and more particularly to a viscous coupling having a variably configurable input rotor allowing control over the transfer of power to an output rotor and to parasitic devices driven by the output rotor.
2. Description of the Problem
Fluid couplings using a viscous working fluid for transmitting torque from an input shaft to an output shaft are well known in the art. Such fluid couplings have typically included an output rotor and a cover which cooperate to define a fluid chamber, a valve plate dividing the fluid chamber into an operating chamber and a reservoir, and an input rotor disposed within the operating chamber and rotatable relative to an output rotor. The input and output rotors define a shear space such that rotation of the input rotor causes viscous fluid to circulate in the shear space and thereby exert a viscous drag on the output rotor, causing the output rotor to rotate. The valve plate defines a fill orifice, and a valving arrangement controls the flow of the working fluid from the reservoir chamber, through the fill orifice, into the operating chamber. When most of the viscous fluid is discharged from the operating chamber to the reservoir chamber the fluid coupling is considered to be “disengaged” and little or no power is transmitted through the device. When viscous fluid partially or fully fills the operating chamber, infiltrating the shear space, the coupling becomes partially or fully “engaged”.
Conventional fluid couplings have exhibited relatively tight clearances between the outer periphery of the input member and the inner periphery of the output member, partly because the viscous fluid between these adjacent peripheries acts as a fluid bearing, and partly to maximize the available shear surface and the torque transmitting capacity. U.S. Pat. No. 4,132,299 taught such a fluid coupling. The '299 patent is also an example of a form of valving to control the flow of fluid into the operating chamber to effect engagement or disengagement.
Conventional fluid couplings have generally been of the type referred to as “full OD”, i.e., the outer surface of the input member and the inner surface of the output member are cylindrical and have a maximum diameter over the entire axial extent of the respective surfaces. A full OD input member provides maximum torque transmission when the fluid coupling is engaged. With the coupling disengaged, however, several problems arise in connection with the use of the full OD input member. One of these is the “cold-start” condition which arises after the coupling has been inoperative for a period of time and fluid has leaked from the reservoir into the operating chamber, causing the coupling to operate as though it were engaged when it is intended to be disengaged. Upon start-up of the coupling under this condition, it typically takes a full minute or more for enough of the fluid to be discharged from the operating chamber back into the reservoir chamber to reduce the speed of the output member to its normal, disengaged level. During this period of time, operation of the coupling may not be desired, e.g., the coupling is driving the radiator cooling fan of a vehicle engine and no cooling is required upon initial start-up of the vehicle engine. A relatively higher disengaged output speed (referred to as “idle speed”) results in a relatively higher horsepower consumption by the coupling and the associated cooling fan with no resultant benefit.
As the interest in improved fuel efficiency in motor vehicles has grown, with the increasing cost of transportation fuel, and the need to comply with emission regimes has been compelled by mandatory government standards, interest has grown in limiting power drawn by parasitic devices which are conventionally driven off the engine or vehicle transmission. Such devices include not a only engine cooling fans, which have routinely been equipped with viscous couplings, but power steering pumps, alternators, compressors, power take-off equipment and even engine superchargers. The ability to disengage such devices when not needed can substantially reduce power demands, thus reducing fuel consumption and potentially easing meeting emission requirements. Also of interest would be controlling unintended engagement of the output rotor stemming from leakage of working fluid from its reservoir into the operating chamber and the ability to provide a rapid responding shear fluid coupling.